Wavelength division multiplexing (WDM) transmission systems mainly include an optical amplifying unit and en optical add-drop multiplexer (OADM). The OADM is provided with a wavelength selective switch (WSS), for example. The WSS can flexibly switch the setting of a route for each wavelength. To compensate loss in the OADM and increase output power to a transmission line fiber, the optical amplifying unit may be arranged before and after the OADM.
Examples of the optical amplifying unit include erbium-doped fiber amplifiers (EDFA) provided with an erbium-doped fiber (EDF). EDFAs can amplify a band of 1530 to 1565 nm (C-band) and 1565 to 1625 nm (L-band). By using the broadband property of EDFAs, WDM transmission systems capable of transmitting signals of 80 or more waves are out to practical use. Furthermore, semiconductor optical amplifiers (SOA) and Raman amplifiers are put to practical use.
An optical amplifying unit includes an EDF, a photo diode (PD), a splitter (SPL), a gain equalizer (GEQ), and a variable optical attenuator (VOA). The EDF provides a gain to a signal having loss in a transmission line. Because the gain provided to each wavelength of the signal by the EDF is not flat, the GEQ flattens the gain. The PD receives a signal split by the SPL to detect the amount of light. The VOA changes the gain provided by the EDF based on the amount of light detected by the PD. To decrease the gain by 1 dB, for example, the VOA increases the loss by 1 dB, thereby varying the gain. Thus, the optical amplifying unit controls the gain provided to the signal.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-156285
Patent Document 2 Japanese Laid-open Patent. Publication. No. 2012-084965
Patent Document 3 Japanese Laid-open Patent Publication No. 2007-028207
The related devices, however, have a large number of components, thereby increasing in size.
The related WDM transmission system includes the optical amplifying unit and the OADM, for example. To vary gain in the optical amplifying unit, the VGA controls the gain, and the OADM controls signal power at each wavelength using a VGA function of the WSS. An influence of the VOA provided to both the optical amplifying unit and the OADM increases optical loss, the size of the device, and cost. Because of the increase in optical loss, a second optical amplifying unit may be added after the OADM. The addition of the second optical amplifying unit further increases the size of the device and the cost.